1. Field of the invention
The present application relates to an optical module.
2. Background Arts
A Japanese Patent laid open No. JP-2010-171922A has disclosed an optical device often called as 90° optical hybrid that includes a 2×4 multi-mode interference (MMI) coupler and a 2×2 MMI coupler. The former MMI coupler converts an optical signal modulated by an algorithm of the quadrature phase shift keying (QPSK), or the differential QPSK, into a first pair of optical signals and a second pair of optical signals, where the optical signals of the first pair are in-phase to each other, and the optical signal of the second pair are also in-phase to each other. The second MMI coupler converts one of the first pair of the optical signals and the second pair of the optical signals into a third pair of optical signals having quadrature phase to the one of the first pair and the second pair.
Recent eager requests to make optical devices smaller and higher in a density thereof have generated an optical hybrid that integrates an MMI coupler with photodiodes (PDs) on a substrate made of indium phosphide (InP). These optical hybrids are often mounted on a base through a carrier made of copper tungsten (CuW). However, differential outputs of an optical hybrid which is selected so as to show equalized differential outputs sometimes increase the equalized difference between the positive and negative phases. The present invention is to provide an optical module that suppresses the increase of the equalized difference between two outputs.